Patent Document 1, for example, discloses a drill which includes such a guide portion. This drill is a long drill having a guide on its land and the guide portion is located at a predetermined distance to the rear from the front of the drill. Also, in such a drill, the guide portion's diameter is smaller than or equal to the drill diameter at the point where the drill touches the face of a drilling hole.
Additionally, Patent Document 2 discloses a cutting edge-tipped drill in which: a tip composed of cutting edge(s) and margin(s), is attached thereto; and a guide pad having a helical or inclined shape, is provided on its shank.
To bore a hole with a drill having such a guide portion:
the cutting edge section in the front of the drill body is fed. and bores a drilling hole; the guide portion is inserted into the drilling hole; then the guide portion slidingly contacts the inner wall face of the drilling hole.
This sliding contact prevents a so-called skipping rope phenomenon from occurring, even if the drill is a long drill as mentioned in Patent Document 1, namely, a drill in which: chip evacuating flutes are formed in the drill body and; the length of the chip evacuating flutes is significantly longer than the external diameter of its cutting edge. The skipping rope phenomenon is a phenomenon in which: while a drill is boring a hole, the drill body between the cutting edge section biting the drilling hole and the shank with which the drill is held to the spindle of a machine tool, vibrates and is deflected radially outward. Therefore, by preventing this phenomenon, the drilling hole can be prevented from curving, whereby linearity of the drilling hole can be ensured.
[Patent Document 1]
    Japanese Unexamined Patent Application No. 2007-196318A[Patent Document 2]    Japanese Utility Model No. S56[1981]-48416